A typical vehicle power system consists of a battery, an alternator to charge the battery and to augment power supplied by the battery, and one or more distribution buses to power a variety of loads connected to the system. In such arrangements there is usually only a single degree of control wherein the field voltage of the alternator is controlled to regulate the output of the alternator and in turn regulate the voltage of the power distribution bus, which is usually on the order of about 14 volts DC or 28 volts DC. This has two primary effects. Firstly, the alternator must operate at a voltage compatible with the battery, even if most of the power provided by the alternator is consumed by a load that is configured to optimally operate at a different supply voltage. In addition, the battery voltage is necessarily the same as the distribution bus voltage regardless of the battery's temperature or state of charge, causing degradation of the battery and reducing its useful life. Further, the charging and discharging of the battery is relatively uncontrolled.
In addition to DC power, portable alternating current (“AC”) power is a desired resource for many specialized vehicles. For example, emergency vehicles may require AC power to operate medical equipment carried on board the vehicle. AC power is likewise used in utility and construction vehicles to operate tools and equipment. Another common use for vehicle-based AC power is in long-haul transport tractor-trailer trucks having a sleeper compartment wherein AC power may be used to operate convenience accessories such as electric razors, televisions and microwaves. Similarly, recreational camping vehicles (“RVs”) use AC power to operate the various household accessories installed in the RV as well as those that may be carried by the passengers.
Static inverters are commonly used to generate portable AC electrical power from a DC power source. Such inverters are relatively lightweight and have no moving parts to wear out. In addition, inverters do not require a fueled engine such as the vehicle's prime mover to produce power, are quiet, and do not produce fumes. Inverters are also more efficient than comparable power sources, such as motor-driven generators. However, inverters suffer from a limitation in that their output power, measured in volt-amps (“VA”) or watts, may be constrained under some conditions. For example, an inverter that derives its input power from a vehicle's alternator system may not be able to deliver the full amount of electrical power demanded by a load when the vehicle is at idle since the power delivery capacity of an alternator varies directly with the vehicle's engine speed.
Aside from the need to generate AC power there is a desire on the part of many vehicle manufacturers to increase the “electrification” of vehicles, i.e., reducing the number of accessories that depend directly on the fueled-engine as a mechanical prime mover. Example accessories include power steering pumps, hydraulic drives, engine cooling fans, air conditioning compressors, oil and coolant pumps, and air compressors. Advantages of accessory electrification include a reduction in engine loading, which facilitates greater engine performance, increased flexibility in locating and mounting the accessories in the vehicle, reduced fuel consumption, more efficient accessory operation made possible by optimizing the location and wiring of the accessories, and reduced vehicle emissions corresponding to reduced engine loading and fuel consumption.
Some vehicles may have several battery power supplies. For example, a vehicle may have a first battery system for operating a starter to “crank,” or start, the engine and a second battery system for powering accessories. The discharge and load characteristics can vary considerably between the cranking and accessory batteries. For example, the cranking batteries are intended to provide high current for a relatively short period of time to start the engine, while the accessory batteries are used to provide a smaller amount of current to the vehicle's accessories for a relatively long period of time. The types of batteries used for cranking and for powering accessories may also differ. For example, a cranking battery may use flooded lead-acid batteries while the accessory battery may use deep-cycle absorbed glass mat (“AGM”) batteries. Each type of battery can have differing charge requirements, as well.
There is a need for an improved way to control battery charging to an amount appropriate for each battery in a vehicle electrical system having multiple batteries. There is a further need for an improved way to control and route power between multiple power supplies and distribution buses in a vehicle in order to supply and augment the buses and charge the batteries.